FIG. 11 is an equivalent circuit diagram of a microwave variable attenuator used as gain control means in a conventional transmitting and receiving module. In FIG. 11, a microwave variable attenuator 1000 includes a transmission line 16 having a characteristic impedance of 50 .OMEGA. and an electrical length of 90.degree.. First to third resistors 15a to 15c each having a prescribed resistance are connected in series between an input terminal 1 and an input end of the transmission line 16. Fourth to sixth resistors 15d to 15f each having a prescribed resistance are connected in series between an output end of the transmission line 16 and an output terminal 2.
A description is given of the operation.
In this microwave variable attenuator 1000, signals input to the input terminal 1 are attenuated while passing through the first to third series resistors 15a to 15c, the transmission line 16, and the fourth to sixth series resistors 15d to 15f and then the signals are output from the output terminal 2. The amount of the attenuation is changed when junctions selected from junctions of the resistors between the input terminal 1 and the transmission line 16 or between the transmission line 16 and the output terminal 2 are connected to each other with a jumper. Therefore, by changing the combination of the two junctions connected with the jumper, the substantial number of the resistors between the input terminal 1 and the transmission line 16 or between the transmission line 16 and the output terminal 2 is changed, whereby the amount of the attenuation is controlled.
Since the electrical length of the transmission line 16 is 90.degree., a reflected input signal at the input end of the transmission line 16 cancels a reflected signal transmitted through the transmission line 16 and reflected at the output end of that line, so that the circuit is free of apparent reflection. More specifically, the wave that is not reflected at the input end of the transmission line 16 but passes through the transmission line 16 and is reflected at the output end of that line is subjected to a phase shift of just 180.degree. as the reflected wave propagates between the input and output ends of the transmission line 16.
In the above-described microwave variable attenuator according to the prior art, the amount of the attenuation depends on the number of the resistors between the input terminal 1 and the transmission line 16 or between the transmission line 16 and the output terminal 2 which are not short-circuited by a jumper and are effectively used as resistors. Therefore, only discrete values of attenuation corresponding to the number of resistors are obtained, so that a precise control of gain (amount of attenuation) cannot be conducted.
Further, since the transmission line 16 having an electrical length of 90.degree. is used to rid the attenuator of apparent reflection due to the transmission line, the size of the device is large.
Furthermore, since the prior art microwave variable attenuator has relatively wide-band characteristics, in order to eliminate unnecessary signals, a special filter for eliminating the unnecessary signals must be added to the attenuator, resulting in a further increase in the size of the device and an increase in the cost.